Dutch patent application 10.03340 of older date and of the same inventor as the present application describes a bipolar battery which has improved performance over existing batteries. More particularly, that earlier patent application describes a bipolar plate basically consisting of a suitable substrate provided on either side with active layers, the substrate basically consisting of a graphite felt filled with a plastic. The contents of that earlier application are considered to be incorporated herein by reference.
The earlier application describes two ways of manufacturing a bipolar plate based on the composite substrate described in that earlier application. A first procedure involves the application of a negative pole paste, known per se, to a main surface of the substrate for forming a negative pole layer, and the application of a positive pole paste, known per se, to the opposite main surface of the substrate for forming a positive pole layer. A second procedure, specifically for the purpose of a bipolar plate for use in a lead-acid battery, involves first manufacturing a lead-plated composite substrate, and subsequently activating the lead-plated composite substrate by carrying out the so-called Plante process in a special manner, whereby the substantially massive lead of one surface is converted to a porous lead grid, while the substantially massive lead of the other surface is converted to a porous grid of PbO2.
It has been experimentally established that the first-mentioned process is able to provide batteries having a good performance. It is a problem, however, to implement this process on a commercial scale, because for that purpose special machines must be developed, a practical problem being that the two opposite surfaces of the substrate must be processed in different ways. More particularly, the two pastes must be formed in different ways. Moreover, it is a drawback that in the application of paste to a flat plate, adhesion is not optimal and the capacity is not optimally utilized; to remove this drawback, the substrate would have to be provided with a grid on both sides.
The second process has, as such, good utility for manufacturing a practically applicable bipolar plate. It has been experimentally established, however, that the performance of the thus manufactured batteries, at high rates of discharge (high discharge current intensities), lags behind the performance of batteries manufactured according to the first-mentioned process. Further, it is a problem that in the Plante process described, a strong corrosion occurs of the thin lead films for forming the porous lead, which corrosion is disadvantageous to the substrate.
The object of the present invention is therefore to provide a method for manufacturing a bipolar plate for, for instance, a bipolar battery, which method can be implemented in a relatively simple and inexpensive manner, while batteries constructed with the thus manufactured bipolar plates have a performance which, in particular at high rates of discharge, is comparable to, or even better than, the performance of known starter batteries.
In starter batteries presently known, use is made not of bipolar plates but of monopolar plates, which are formed as thin, pasted plates. In manufacturing monopolar plates, use can be made of a separate manufacturing machine for positive plates, as well as of a separate manufacturing machine for negative plates, that is, the positive plates and the negative plates are manufactured independently of each other. This provides the advantage that the manufacturing processes used for manufacturing positive plates and for manufacturing negative plates can be optimized independently of each other.
It is customary to manufacture monopolar plates in the form of a metal grid in which a paste is applied, which paste is subsequently formed. During the forming operation, the metal grid serves as a matrix to keep the paste in position. During use, the metal grid serves on the one hand to give the plate mechanical integrity and on the other for current conduction. Because in monopolar plates the current is drawn via a side edge of the plate, which means that the direction of the electric current within the plate is parallel to the plate surface, the metal grid should be of rather heavy design. In a practical example, the grid is constructed in the form of horizontal and vertical bars whose thickness is in the order of a few millimeters, which bars enclose rectangular spaces of about 5xc3x9710 mm2, in which spaces the paste is applied. The mass ratio of the paste to the grid is approximately 1:1.
More specifically, therefore, the object of the invention is to provide a method for manufacturing a bipolar plate, as well as a bipolar battery and a method for manufacturing same, in which the advantages of existing techniques are combined as much as possible, while the disadvantages of existing techniques are eliminated as much as possible.
According to an important aspect of the present invention, in a method for manufacturing a bipolar plate, use is made of separately manufactured pasted plates, which are applied to the substrate or pressed onto it. This has the advantage, in the first place, that use can be made of existing techniques for manufacturing pasted plates. However, because in a bipolar plate the current is drawn in a direction perpendicular to the surface, the current conducting properties of the grid of the pasted plates (for current conduction in a direction parallel to the surface) need to meet less stringent requirements. Moreover, the pasted plates are supported over their entire surface by the substrate, so that they do not need to be self-supporting anymore. The above-mentioned two aspects imply that the metal grid of the pasted plates can be considerably less heavy than in the case of monopolar plates, which means that a greater percentage of the weight of the pasted plates can consist of paste, so that the performance per unit of weight improves. A paste-to-grid weight ratio of 3:1 seems realizable without any problems, but even a ratio of 10:1 is envisaged.
A further aspect of the present invention is directed to the provision of a substrate, as well as to the provision of a method for manufacturing a substrate, which substrate is better suited than known substrates to be used in combination with pasted plates placed against it. According to this further aspect of the present invention, the substrate is provided with a lead film, in order to ensure the best possible contact between the substrate on the one hand and the pasted plates on the other. For a good quality of the lead films, it is preferred that the substrate is first thermally pressed, and the thermally pressed substrate is subsequently subjected to a surface activating treatment, whereafter the lead film is applied.